A number of different types of pump assemblies are utilized for recovering liquid from subterranean areas, such as crude oil from an oil well shaft or bore hole. These include walking beam pump apparatus and electric motor and pump assemblies that may be positioned above ground so as to lift the oil from the subterranean producing strata through a production string. Alternatively, a pump and motor assembly may actually be positioned in the shaft near the producing strata in a working barrel connected to the production string. In operation the pumps remove liquid from the shaft or cavity surrounding the intake pipe at the end of the production string and pump the liquid through the production string to a storage tank or other facility where it is held for further processing.
Typically, pumps of the types described have a pumping capacity that exceeds the rate at which oil and liquid flows from the producing strata into the shaft. As a result, continuous operation of the pumps causes the shaft surrounding the intake pipe to be emptied of liquid. Of course, operation of a pump in a cavity or shaft emptied of liquid is undesirable. It results in unnecessary and excessive pump wear and possible damage. The energy used to operate the pump is also effectively wasted and production efficiency is significantly reduced.
Recognizing these problems, a number of systems have been developed for providing intermittent operation of a pump when the cavity surrounding the intake pipe is filled with the desired level of liquid.
In one approach, the operation of the pump is controlled exclusively by a timer. The rate of liquid production from the producing strata is studied to determine the best timer settings. For example, the timer may be adjusted so that the pump is operated every four hours for a period of twenty minutes. As long as the producing strata continues to produce liquid at a substantially constant rate, this type of pump apparatus may be operated efficiently. However, it should be appreciated that often the rate of liquid production from producing strata varies from season to season, month to month, and even day to day. As a result, a pump apparatus exclusively controlled by a timer must be carefully monitored in order to ensure that the pump is being operated at maximum efficiency: that is, only when sufficient liquid is present in the cavity to allow the desired pumping. Unfortunately, this is an inconvenient, labor intensive and time consuming task.
An alternate approach providing intermittent operation of a pump includes the utilization of a pair of liquid level sensors that are positioned in the oil well shaft. One level sensor is positioned at the maximum desired liquid level and the other sensor is positioned at the minimum desired liquid level in the shaft. When the liquid level in the shaft reaches the upper or maximum level sensor, a switch is closed and pump operation is initiated to recover liquid from the shaft. Once the liquid level drops just below the lower or minimum level sensor, pump operation is discontinued. Such a pump control system is described and claimed in, for example, U.S. Pat. No. 3,132,592 to Rudy et al.
While a level sensor control apparatus of the type disclosed in the Rudy et al patent is effective in providing more efficient operation of the pump even when liquid production rates from the strata vary to a significant degree, the apparatus is not without its drawbacks. The primary concerns relate to overall reliability. The two level sensors/switches must be carefully mounted along the production string at desired locations. The string must then be carefully lowered into the oil well shaft. During lowering, the switches may come into contact with the side of the shaft. If the position of either of the switches is moved relative to the other, the operational efficiency of the pump may be adversely effected. Alternatively, one or both of the switches may, in fact, be damaged through, for example impact with the side wall of the shaft. If this occurs, the pumping control system is effectively made inoperative.
It should also be appreciated that, even when properly installed, the level sensors/switches are exposed to severe operating conditions in the oil well shaft. Subterranean pressure conditions may at times reach 1,000 psi or more. In addition, the sensors/switches are often contacted by chemicals leached from the surrounding strata. Often, strong acids are released which over time have a deleterious effect on switch operation. Again, as mentioned above, if either switch becomes damaged, the control system is effectively rendered inoperative. Of course, because of the exposure of the switches to the severe elements in the oil well shaft, the chances of one of the switches becoming inoperative over time are significant.
Yet another alternative pump control system includes a single level sensor switch providing a timed operation of the pump. Such a device is described and claimed in U.S. Pat. No. 3,413,429 to Yost. The Yost patent discloses a switch including two chambers each having a diaphragm. As liquid from the producing strata enters the oil well shaft, the pressure exerted on the diaphragm of the first chamber increases forcing fluid within the first chamber through a one-way valve into the second chamber. This causes a switch to close and the initiation of the pumping operation. As liquid is recovered from the shaft, the pressure exerted on the diaphragm of the first chamber decreases. As a result, the pressure of the fluid in the first chamber decreases. Eventually fluid pressure in the second chamber is sufficient to overcome the force of a bleed-off valve spring. Fluid flows through the bleed-off valve from the second chamber to the first chamber until pressures in the two chambers are equalized. At that point in time pump operation is terminated.
While the control apparatus provided for in the Yost patent functions to provide effective operation of the pump, it should be appreciated that the structure provided for the switch is unduly complicated and somewhat unreliable. For example, the diaphragm of the first chamber is exposed to the harsh environment of the oil well shaft. It could either be damaged as, for example, by puncturing during positioning of the production string in the shaft or by the acidic and corrosive chemicals leached from the surrounding strata.
It should further be appreciated that the pump control described in the Yost patent does not provide the desired flexibility to allow the well operator to maintain maximum production efficiency in response to changing conditions. More particularly, the strenth of the spring of the bleed-off valve determines the length of time the pump is in operation. Since the bleed-off valve spring is located underground in the shaft, it cannot be readily changed or adjusted. As such, the operation of the pump cannot be adjusted to, for example, recover a different volume during a particular pumping cycle such as when necessary to maintain a certain static pressure above a producing zone. The device can also not be adjusted to meet changes in desired production quotas. As such, a need is identified for an improved oil well pump control system.